Method of manufacturing medical electrical lead

ABSTRACT

A method of manufacture for a medical electrical lead, and in particular, a method of manufacturing a sintered porous platinized electrode used in a steroid eluting pacing cardiac lead which offers increased dimensional and shape consistency over previously used manufacturing methods. The method of manufacturing the electrode consists primarily of mixing a conductive material and a binder to form a slurry, introducing a distal end of a substrate into a mold, creating a negative pressure in the mold, injecting the slurry into the mold to form a mass of conductive material on the substrate, and sintering the mold, substrate and conductive material to form an electrode.

This is a continuation of U.S. patent application Ser. No. 08/146,265,filed Oct. 29, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to medical electrical leads and methodsof manufacture thereof and, in particular, to a method of manufacturinga sintered porous platinized electrode used in a steroid eluting pacingcardiac lead which produces an electrode having increased dimensionalconsistency over previously used manufacturing methods.

2. Description of the Prior Art

The safety, efficacy and longevity of an implanted pacemaker systemdepends, in part, on the performance of the pacing leads, the electroniccircuits and the integrity of the pulse generator, and the capacity andreliability of the pulse generator power source. These inter-relatedcomponents of an implanted pacemaker system optimally are matched in afashion that accommodates ever increasing demands on the modes ofoperation and function of the system in conjunction with an overallreduction in system size, an increase in system longevity and anincreased expectation in system reliability.

During the past thirty years, the technology of cardiac pacing hassignificantly advanced. Implantable pacing systems offer an everincreasing variety of pacing modalities, thereby substantiallybroadening the indications for pacemaker use. In conjunction with thisadvancement, there has been extensive research and development tooptimize the performance of pacing leads and their reliability whileconcurrently simplifying their manufacture.

In the past ten years, substantial improvements in chronic pacemakersensing and stimulation thresholds have been achieved which, in turn,have allowed the development of smaller and longer-lived pacemakers thatcan be used with smaller leads. As new circuits are developed with lower"overhead" current drains, however, and as the circuits increase incomplexity to allow for ever increasing pacemaker capabilities in theirprogrammable functions, modes and memory, pacemaker longevity dependsincreasingly more on the characteristics of the lead. In addition, manydoctors prefer pacing leads be made ever thinner, to occupy less spacein the venous system, without diminishing or detracting from themechanical strength and integrity of the lead body.

Recently, various investigators have emphasized materials and theirrelationship to the considerations involved in optimizing electrodedesign. For example, Bornzin, U.S. Pat. No. 4,502,492 owned byMedtronic, Inc. discloses a low polarization, low threshold electrodedesign which was commercialized as the TARGET TIP® lead during the earlyto mid-1980's. That design featured a generally hemispherical electrodewith circular grooves, fabricated from platinum and coated over itsexternal surface with a plating of platinum black. This combination ofthe relatively low (8 mm²) macroscopic electrode surface area andrelatively high microscopic electrode surface area (due to the use ofplatinum black) contributed to the achievement of state-of-the-artthresholds for that time period. Other manufacturers marketed electrodesof other materials and configurations including totally porous platinummesh (Cardiac Pacemakers, Inc.), porous surface sintered (CordisCorporation), glassy and vitreous carbons (Siemens Inc.), and laserdrilled metal (Telectronics Ltd.) electrodes in that same time period.

A considerable breakthrough in the development of low thresholdelectrode technology occurred with the invention of the steroid elutingporous pacing electrode of Stokes, U.S. Pat. No. 4,506,680 and relatedMedtronic U.S. Pat. Nos. 4,577,642; 4,606,118 and 4,711,251, allincorporated herein by reference. The electrode disclosed in the Stokes'680 patent was constructed of porous, sintered platinum or titanium,although carbon and ceramic compositions were also mentioned. Proximatethe electrode a plug of silicone rubber impregnated with the sodium saltof dexamethasone phosphate, or a water soluble form of otherglucocorticosteroids, was placed. The silicone rubber plug allowed therelease of the steroid through the interstitial gaps in the poroussintered metal electrode to reach the electrode-tissue interface andprevent or reduce inflammation, irritability and subsequent excessivefibrosis of the tissue adjacent to the electrode itself. The poroussteroid eluting electrode presented a sensing impedance substantiallylower compared to similarly sized solid electrodes and presentedsignificantly lower peak and chronic pacing thresholds than similarlysized solid or porous electrodes. The advantages of the steroid elutingelectrode allowed a relatively small surface area electrode of about 5.5mm² (CAPSURE® SP Model 5023, 5523 leads sold by Medtronic, Inc.) toraise lead impedance without sacrificing the ability to sense heartactivity.

The smaller electrode size was important because it resulted in highercurrent density during stimulation pulses. This, in turn, was importantbecause it provided more efficient stimulation of the heart tissue withlower current drain from the implanted pacemaker power source.

This resulted in overall increased longevity of the implanted pacemakersystem.

Lead impedance is a function of the resistance of the lead conductor andthe stimulating electrode as well as the effective impedance of theelectrode-tissue interface. An inefficient way to raise impedance is toincrease the resistance of the lead conductor. This wastes current asheat. It is preferable to decrease lead current drain with moreefficient control of the stimulating electrode-tissue interfaceimpedance. This can be done by reducing the geometric surface area ofthe electrode.

Recent advances in lead design have continued decreasing the exposedgeometric surface area of the electrode. One such example is disclosedin Stokes et al. U.S. patent application Ser. No. 07/887,560 filed May18, 1992 now U.S. Pat. No. 5,282,844 entitled "High Impedance, LowPolarization, Low Threshold Miniature Steroid Eluting Pacing LeadElectrodes" assigned to the assignee of the present inventionincorporated herein by reference, which discloses a lead featuring aporous platinized steroid eluting electrode exhibiting an effectivesurface area in the range of 0.1 to 4.0 mm², and preferably between 0.6to 3.0 mm² which offers increased pacing impedance without increasingthresholds and without negatively impacting sensing capabilities overpreviously lead, and in particular, electrode designs. The particularlead disclosed in the before-mentioned application offers a pacingimpedance of at least 1400±260 ohms, and a source impedance of at least1650±410 ohms. Such a lead, however, due to its size requires anelectrode which is relatively more difficult to consistentlymanufacture, especially with respect to electrode diameter andconcentric dimension.

Previously, porous platinized steroid eluting electrodes were sometimesmanufactured using a slurry-drip process. Specifically a slurry mixtureof platinum particles and a liquid organic binding agent was created.

To form the electrode, a substrate, typically a straight shank of wire,had the slurry mixture dripped onto one end of the substrate. A portionof the slurry mixture stuck to the substrate. After several applicationsof the slurry mixture (between 10 to 50 separate applications) anacceptable mass of the mixture is built up on the end of the substrate.The substrate is then sintered to drive off the binder and fuse theplatinum particles together. This manufacturing process generally yieldsan electrode having acceptable electrical characteristics.

With the ever smaller size of cardiac leads, however, dimensionalvariance of the electrode has a relatively greater impact. In particularthe slurry-drip method of electrode manufacture has been found to notoffer the dimensional consistency desired in present-day relativelysmaller leads. Because leads are smaller than ever before, the samedimensional variance in electrode size now has a relatively largereffect upon lead quality than it did when leads were relatively larger.Specifically, under conditions of mass production the resultantelectrode is many times dimensionally inconsistent, e.g. eccentric, toolarge or too small and thus unacceptable. Such inconsistency increasesthe number of rejected leads, and thus results in higher manufacturingcosts.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide a medicalelectrical lead and method of manufacture, and specifically an electrodeused in such a lead, which has increased dimensional consistency overpreviously provided leads and methods of manufacture.

The present invention concerns a medical electrical lead, and inparticular to a method of manufacturing a sintered porous platinizedelectrode used in a steroid eluting pacing cardiac lead which offersincreased dimensional consistency over previously used manufacturingmethods. The method of manufacturing the electrode of the presentinvention permits the electrode to be more consistently manufacturedwith respect to its dimensions and desired shape characteristics. Themethod of the present invention also permits such an electrode to beconstructed using a substrate tailored to achieve the desired impedance.For example in one embodiment of the present invention an electrode isconstructed with a nailhead-shaped substrate which provides for a higherimpedance lead to be constructed. The method of manufacturing theelectrode portion of a medical electrical lead of the present inventionbasically comprises the steps of mixing a conductive material and abinder to form a slurry; providing an electrode substrate within a mold;creating a negative pressure in the mold; injecting the slurry into themold; releasing the negative pressure in the mold; sintering the mold,conductive material and substrate, to drive off the binder and hardenthe conductive material.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention may befully understood and appreciated in conjunction with the attacheddrawings and the following detailed description of the preferredembodiment where the same numerals are employed to denote the same orsimilar features throughout.

FIG. 1 is a plan view of an endocardial, unipolar pacing lead accordingto the present invention;

FIG. 2 shows a cross-sectional view of the distal end of the lead shownin FIG. 1;

FIG. 3 shows a cross-sectional view of a mold used to form an electrodeused in a lead of the present invention;

FIG. 4 shows a cross-sectional view of the mold shown in FIG. 3 takenalong the line 3--3;

FIG. 5 shows an electrode constructed according to the present inventionfor use in a medical electrical lead and featuring a nail-head shapedsubstrate; and

FIGS. 6 and 7 illustrate the various dimensions of an electrodeconstructed according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

As a general comment, the present invention preferably includes the useof a steroid or other drug with the electrode. The electrode may beconfigured to allow the drug to be eluted through and/or around theelectrode in order to reach the endocardial or myocardial cells near thedistal end of the pacing lead in order to reduce, if not eliminateentirely, the inflammation and irritation caused by the presence of alead and especially in response to the tip of a lead. As described inStokes, U.S. Pat. No. 4,506,680 and related Medtronic U.S. Pat. Nos.4,577,642; 4,606,118 and 4,711,251, mentioned above, the electrode ispreferably fabricated from a body compatible electrically conductivematerial with or without specific steroid eluting passages but generallywith a porous structure either throughout the body of the electrode orat its surface. The porosity of the electrode surface or body provides alarge surface area for sensing whereas the overall dimension or shape ofthe exposed electrode defines a comparatively smaller surface area forstimulation. The porous structure thus presents a relatively largemicroscopic (or "fractal") surface area for sensing and a relativelyvery small macroscopic or geometric surface area for stimulation.Acceptable electrode materials and the associated fabrication techniquesemployed to achieve the micro-porous structure, as well as the porosityof that structure are all set forth in the aforementioned prior artpatents and in the Richter et al., U.S. Pat. No. 4,773,433; Heil Jr. etal., U.S. Pat. No. 4,819,661; Thoren et al., U.S. Pat. No. 4,149,542;Robblee, U.S. Pat. No. 4,677,989; Heil Jr. et al., U.S. Pat. No.4,819,662; Mund et al., U.S. Pat. No. 4,603,704; Skalsky et al., U.S.Pat. No. 4,784,161; Szilagyi, U.S. Pat. No. 4,784,160, hereinincorporated by reference.

The present invention concerns an electrode used in a medical electricallead, and in particular to a method of manufacturing an electrode usedin a sintered porous platinized electrode used in a steroid elutingpacing cardiac lead which offers increased dimensional and shapeconsistency over previously used manufacturing methods. The method ofthe present invention also permits such an electrode to be constructedusing a substrate tailored to achieve the desired impedance. For examplein one embodiment of the present invention an electrode is constructedwith a nailhead-shaped substrate which provides for a higher impedancelead to be constructed.

LEAD

FIG. 1 illustrates a plan view of an endocardial, unipolar leadconstructed in accordance with the present invention. The lead 1includes an elongated lead body 10 covered by an insulative sleeve 12.Insulative sleeve 12 may be fabricated of any flexible biocompatible andbiostable insulator especially silicone rubber or polyurethane. At theproximal end 2 of lead 1, terminal assembly 14 is adapted to couple lead1 to an implantable pacemaker pulse generator (not shown.) Terminalassembly 14 is provided with sealing rings 16 and a terminal pin 18, allof a type known in the art. An anchoring sleeve 20 (shown partially incross-section) slides over lead body 10 and serves as a point forsuturing lead body 10 to body tissue at the insertion point of lead 1 ina fashion known in the art. Anchoring sleeve 20 and terminal assembly 14are preferably fabricated of silicone rubber, although they may also beconstructed of any other suitable biocompatible material known in theart.

Lead 1, as shown in FIG. 1, further includes a stylet guide 11 andstylet assembly 13 coupled to terminal pin 18 for imparting stiffness tolead 1 during the insertion and placement of lead 1 transvenously intoeither the right ventricle or the right atrium of the heart (not shown.)Stylet guide 11 and stylet assembly 13 are discarded after use andbefore connection of terminal pin 18 to a pacemaker pulse generator (notshown.)

At distal end 3 of lead 1, a tine protector 15 is shown (incross-section) protecting tine assembly 38 having a series of tines 26until lead 1 is used. Tines 26 are employed to passively retainelectrode 22 in position against the endocardium (not shown) as is wellknown in the pacing art.

FIG. 2 shows in cross-section proximal end and distal end of lead 1 ofthe present invention. As seen lead 1 includes conductor coil 28extending throughout, i.e., from terminal pin 18 to electrode 22.Preferably conductor coil 28 is multifilar in construction. Substrate 23is depicted as a substantially straight piece of a conductive material,such as a platinum alloy, although substrate may be formed into othershapes, such as having a nail-head shaped distal end 27, as best seen inFIG. 5. A substrate having a nail-head shaped distal end 27 has beenfound to offer a somewhat higher impedance electrode than thosefeaturing a straight shank substrate. Nail-head shaped distal end 27 maybe formed in any manner, such as through heat, including actuallymelting distal end 27 to the desired shape, mechanical deformation, oreven molding substrate having such a distal end. Any shape, however,functioning to increase impedance to the electrode 22 and thus lead 1may be used. In addition, the overall length of electrode 22 may beshortened and a series of holes 47 may be provided radially about raisedsurface 24, as best seen in FIG. 5, to provide for an increased elutionrate of steroid from steroid-silicone compound ring 40 (discussed below)to the distal tip of electrode 22. Further discussion and disclosureregarding other shaped electrode substrates may be found in UnitedStates patent application of Gates entitled "Substrate For A SinteredElectrode", filed Apr. 30, 1993 now U.S. Pat. No. 5,408,744, assigned tothe assignee of the present invention and incorporated herein byreference.

Electrode 22 is depicted as a porous platinum object covered withplatinum black at the end of substrate 23. Although platinum is thepreferred material for electrode 22 and substrate 23, they mayadditionally include or be made entirely from various other materials,including but not limited to such materials as palladium, titanium,tantalum, rhodium, iridium, carbon, vitreous carbon and alloys, oxidesand nitrides of such metals or other conductive materials. Of course,some materials are incompatible with others, such as a platinumsubstrate with a titanium electrode, and may not be effectively usedtogether. The limitations of specific materials for use with others iswell known in the art. Moreover, although in the preferred embodimentelectrode 22 is manufactured from a spherical platinum powder, otherforms of conductive materials besides spherical may be used, includingsuch forms as fines, fibers or polyhedrons.

Substrate 23 extends from electrode 22 to distal end of conductor coil28 where it is attached to proximal end 24 of substrate 23 by crimpingat point 34 of crimping member 36 at the time of manufacture. Anadhesive, such as a silicone medical adhesive, may be used at variouspoints 32 to seal against leakage of fluid, such as blood, intoconductor coil 28. Insulative sleeve 12 is placed over skirt 33 andcrimping member 36 as well as proximate tine assembly 38.Steroid-silicone compound ring 40 is located proximate to electrode 22.

Steroid-silicone compound ring 40 forms a monolithic controlled releasedevice when it is loaded with an anti-inflammatory agent, e.g., asteroid dexamethasone sodium phosphate. The steroid also is depositedwithin the pores of electrode 22 by application of a solution of 200 mgU.S.P. dexamethasone sodium phosphate dissolved in 5.0 cc isopropanoland 5.0 cc distilled or deionized water as described in theaforementioned Stokes' patents. Weight and composition ofsteroid-silicone compound ring 40 as well as the electrode surface areaare critical to the overall performance of electrode 22. In particularsteroid-silicone compound ring 40 is positioned proximate said electrodeto dispense a drug in the vicinity of body tissue.

In a preferred embodiment electrode 22 has a macroscopic surface area ofless than 4.0 mm² exposed to the body tissue or fluids or both and morepreferably, but not limited to, in the range of 0.10 and 4.0 mm². Thesurface of electrode 22 exposed to the body tissue or fluids or both isgenerally hemispherical. The small geometric macroscopic electrode sizeis intended to produce very high pacing impedance. The porous surfaceconfiguration together with platinum black electroplating and steroidcontribute to a microscopically large surface area for low polarization,low source impedance and low thresholds. The porous surface alsofacilitates the retention of steroid and adhesion of the platinum blackto the electrode surface. The electrode 22, therefore, permits steroidto elute therethrough. The electrode 22 and lead, especially tineassembly 38, may preferably be dimensioned, moreover, to further allowsteroid to elute around the electrode 22, i.e., between electrode 22 andtine assembly 38.

MOLD

Mold 50 is constructed from two identical halves 51, 52 separable alongmold-line 53, as seen in FIG. 3. Preferably mold 50 is constructed froma rigid material having a high melting point so as to permit mold 50 towithstand the heat if electrode 50 is sintered in an oven. In thepreferred embodiment mold 50 is constructed from graphite, althoughother materials having a high melting point may be used.

Provided within mold 50 is mold cavity 54 which conforms to the desiredshape of electrode 22. Substrate port 55 permits substrate 23 to beinserted into mold cavity 54, as seen in FIG. 4 in phantom outline 60,and permits the conductive material to be applied thereon.

Suction groove 61 connects with suction port 62 to permit a negativepressure, such as a vacuum, to be created in mold cavity 54. Suctiongroove 61 is dimensioned so as to have a width of less than the width ofwhatever conductive material is used. For example in a preferredembodiment of a lead, an electrode constructed from a spherical platinumpowder having an average particle diameter of 0.0011 inches would beused with a suction groove 61 having a width of 0.0008 inches. In such amanner a negative pressure using suction groove 61 would not permitconductive material to flow within and clog suction port 62.

Mold cavity 54 is also constructed having injection port 63 openingwithin. Injection port 63 thereby permits a conductive material to beinjected into mold cavity while a negative pressure, such as a vacuum,is created therein through suction port 62 and suction groove 61.Through the concurrent creation of a negative pressure while theconductive material is injected into the mold cavity 54 a complete fillof mold cavity 54 is accomplished, which, in turn, because mold isrigid, ensures a properly shaped and dimensioned electrode to bemanufactured.

METHOD OF MANUFACTURE

As discussed above, electrode 22 is manufactured by a process ofinjecting a mixture into mold cavity while, preferably, concurrentlycreating a negative pressure, such as a vacuum, with mold cavity 54.

The first step of the manufacturing process comprises mixing aconductive material and a binder to form a conductive slurry mixture. Ina preferred embodiment, conductive mixture comprises 70 weight percentof a spherical platinum powder and 30 weight percent of a bindersolution. The preferred binder solution consists of 2 percent of anorganic binder, such as KLUCEL™ manufactured by Aqualon Corp. ofWilmington, Del. and 98 percent deionized water. The relativeproportions of these constituents has been found to influence theultimate porosity of the electrode produced. Specifically, the greaterthe amount of binder to conductive material, the relatively higherporosity electrode produced.

The next step comprises introducing distal end 25 of substrate 23 into amold 50, and specifically into mold cavity 54 as depicted by phantomline 60 of FIG. 4.

Next, a negative pressure, such as a vacuum, is created in mold cavity54 through suction port 62 and suction groove 61. Then, while undernegative pressure, the conductive mixture is injected into mold cavity54 so as to substantially fill mold cavity 54 and form a mass ofconductive material on distal end 25 of substrate 23. In the preferredembodiment a negative pressure of between 20 to 27 inches of mercury isused. The amount of pressure used, however, depends, in part, upon thedesired porosity of the electrode to be formed.

Once formed, the mixture and substrate are sintered, as is well known inthe art, to draw off the binder and harden the conductive material tosubstrate 23. In a preferred embodiment, sintering is accomplished byplacing the entire assembly, viz. mold 50 having substrate 23 thereinand mass of conductive material on distal end 25 into a vacuum oven at2475 degrees fahrenheit for one hour. Sintering may further beaccomplished through any other methods suited to harden the conductivematerial to substrate 23 while removing binder, such as electricalsintering.

Next, the mold 50 is removed from the sintering device, an oven in thepreferred embodiment of the present invention, and electrode 22 isremoved from mold 50. Any runner formed through solidification ofmixture material within injection port is then removed to give electrodea relatively uniform shape.

Electrode 22 is then preferably electroplated with a material to providea relatively high microscopic surface area, such as platinum black inthe preferred embodiment. Electroplating may be accomplished in anymanner suitable. In the preferred embodiment, electroplating ofelectrode 22 with a platinum black material is accomplished as follows.First, to assure good adhesion of the platinum black electrode 22 iscleaned. After suitably cleaning, electrode 22 may be platinized byimmersing electrode 22, as cathode, in a platinizing solution, such asone consisting of three percent (3%) platinum chloride dissolved in0.025 percent lead acetate solution. An anode of inert metal may then beplaced into the platinizing solution and a sufficient current passedthrough the cell so that small bubbles are visible at the electrode 22.This process should be continued until a layer of platinum black isdeposited over the entire electrode. This process, cleaning andplatinizing, produces an electrode having a platinum black surfacecoating which is sufficiently durable to permit it to be implantedwithin a body. The porosity of electrode 22, together with the platinumblack coating is intended to reduce source impedance and polarization,as is well known in the art. The final step comprises providing orassembling electrode 22 into a medical electrical lead 1, such as oneshown in FIG. 1.

FIGS. 6 and 7 show various dimensions A'-D' of an electrode constructedaccording to the present invention. Shown below is a comparison ofmeasurements and the standard deviations (in parentheses) for electrodesmade using the prior art slurry-drip process and the process of thepresent invention.

    ______________________________________                                        Electrode   METHOD USED                                                       Dimension   Slurry-Drip       Present-Invention                               ______________________________________                                        A'          0.0374  (0.0015)  0.0366                                                                              (0.0002)                                  B'          0.0370  (0.002)   0.0360                                                                              (0.001)                                   C'          0.053   (0.010)   0.247 (0.0025)*                                 D'          0.001   (0.003)   0.001 (0.0001)                                  ______________________________________                                         *Note: difference size produced.                                         

Thus it is seen that relatively small electrodes may be moreconsistently constructed and manufactured according to the presentinvention which satisfy the aforementioned desirable characteristics ofa pacing lead, i.e. low stimulation thresholds, relatively lowpolarization, good to excellent sensing, and adequately low sourceimpedance. Higher pacing impedance prolongs the longevity of pacingpulse generators and allows for the miniaturization of their components.

While the embodiments of the present invention have been described inparticular application to cardiac pacing, and in particular toendocardial pacing leads it will be understood the invention may bepracticed in other electrode technologies where the aforementionedcharacteristics are desirable, including epicardial leads as well asneurological and muscle stimulation applications.

Moreover, although the invention has been described in detail withparticular reference to a preferred embodiment and alternate embodimentsthereof, it will be understood variations and modifications can beeffected within the scope of the following claims. Such modificationsmay include substituting elements or components which performsubstantially the same function in substantially the same way to achievesubstantially the same result for those described herein.

What is claimed is:
 1. A method of manufacturing a medical electricallead comprising:providing an electrical conductor having a first end anda second end; providing an insulating sleeve around said electricalconductor and in a portion between said first end and said second end ofsaid electrical conductor; providing a connector coupled to said firstend of said electrical conductor; providing an electrode coupled to saidsecond end of said electrical conductor for conducting electrical energyto and from a body, said electrode manufactured by the methodcomprising: mixing a conductive particulate material and a binder toform a slurry, said conductive particulate material having a firstdimension; introducing a substrate having a distal end into a mold;creating a negative pressure in said mold through a suction opening, thesuction opening having a second dimension, the second dimension lessthan the first dimension; injecting said slurry into said mold to form amass of conductive material on said distal end of said substrate;heating said mold having said substrate and said mass of conductivematerial on said distal end of said substrate to thereby sinter theparticulate material together on said distal end of said substrate andform a porous electrode; removing said porous electrode from said mold;and coupling said proximal end of said substrate to said conductor. 2.The method according to claim 1 wherein said electrode formed is porous.3. The method according to claim 2 wherein said conductive material isfines.
 4. The method according to claim 1 wherein said conductivematerial is selected from the group consisting of platinum, palladium,titanium, tantalum, rhodium, iridium, carbon, vitreous carbon andalloys, oxides and nitrides of such metals.
 5. The method according toclaim 1 wherein said conductive material is spherical.
 6. The methodaccording to claim 1 further comprising a drug dispenser positionedproximate said electrode to dispense a drug to said body.
 7. The methodaccording to claim 6 wherein said drug is an anti-inflammatory agent. 8.The method according to claim 6 wherein said drug is a salt ofdexamethasone phosphate.
 9. The method according to claim 6 wherein saiddrug dispenser comprises a water permeable polymer body located withinsaid insulating sleeve and adjacent said electrode containing a watersoluble form of said drug.
 10. The method according to claim 1 whereinsaid sintering step comprises heating said mold having said substrateand said mass of conductive material on said distal end of saidsubstrate in an oven.
 11. The method according to claim 1 furthercomprising the step of coating a surface of said electrode with platinumblack.
 12. A method of manufacturing a medical electrical leadcomprising:providing an electrical conductor having a first end and asecond end; providing an insulating sleeve between the first end and thesecond end of the electrical conductor the insulating sleeve providedaround the electrical conductor; providing a connector coupled to thefirst end of the electrical conductor; providing an electrode coupled tothe second end of the electrical conductor for conducting electricalenergy to and from a body, the electrode manufactured by the methodcomprising: mixing a particulate conductive material and a binder toform a slurry, the particulate conductive material having a firstdiameter; introducing a substrate having a distal end into a mold;creating a negative pressure in the mold through a first port, the firstport having a first dimension, the first dimension being less than thefirst diameter of the conductive material; injecting the slurry into themold to form a mass of conductive material on the distal end of thesubstrate; heating the mold having the substrate and the mass ofconductive material on the distal end of the substrate to thereby sinterthe particulate material together on the distal end of the substrate andform a porous electrode; removing the porous electrode from the mold;and coupling the proximal end of the substrate to the conductor.
 13. Themethod according to claim 1 wherein the particulate conductive materialis spherical platinum.